1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) apparatus for obtaining tomographic images, metabolite distribution, or metabolite images of an interior of a body to be examined by utilizing a nuclear magnetic resonance (NMR) phenomenon, incorporating a function for measuring a temperature increase due to an application of radio frequency magnetic fields.
2. Description of the Background Art
In order to shorten the image data acquisition time in the MRI, an imaging method for applying a plurality of radio frequency (RF) magnetic fields onto a body to be examined within a short period of time is often employed. However, when many RF magnetic fields are applied according to this imaging method, a temperature inside a living body increases due to the induced heating phenomenon.
For this reason, the Food and Drug Administration (FDA) of U.S.A. proposed the first safety standard for the MRI apparatus in 1982 in order to secure the safety of the MRI apparatus with respect to living bodies. This proposal recommends to limit an application of RF power onto a living body according to the specific absorption ratio (SAR). According to the guideline at that time, It was recommended to keep the SAR below 0.4 W/kg for a whole body on average, and below 0.2 W/kg per 1 g of any body part locally. This standard was revised in 1988 to recommend the SAR below 0.4 W/kg for a whole body on average, below 8 W/kg for 1 g of tissue, and 3.2 W/kg for the head. At the same time, it was additionally proposed to keep the body temperature increase below 1.degree. C. at the body center portion, and the temperature below 38.degree. C. at the head, below 39.degree. C. at the trunk portion, and below 40.degree. C. at the hand and leg portions.
However, after the RF power is determined according to the SAR prior to the pulse sequence execution, the pulse sequence is executed regardless of a beat generation state of a body to be examined so that it has been impossible to confirm the safety of a body to be examined.
In addition, the temperature increase in an interior of a body to be examined varies considerably depending on an observed body portion, an observed tissue, and a blood flow state, so that it is impossible to comprehend an actual state of the temperature increase in an interior of a living body from a standard value of the SAR alone. For this reason, even when the pulse sequence is executed by using the applied RF magnetic field power determined in accordance with the SAR standard, there has been a possibility for causing a damage to a body to be examined due to an unexpected temperature increase induced in a body to be examined.
On the contrary, there is also a possibility for overestimating an amount of heat generation, and there have been cases in which the data acquisition cannot be carried out efficiently because the sufficient RF magnetic field power cannot be applied despite of the fact that an excessive heat generation is not likely to occur.